Addiction may be the motive.

DAME Janet Smith's report gives the most likely explanation yet as to the serial killer's motive for murder.

The former GP has never offered a reason for his murderous ways and still denies committing the crimes.

In her inquiry, Dame Janet Smith looked at Shipman's history of drug abuse during the early 1970s - he injected the painkiller pethidine - in an attempt to establish a motive.

He was fined in 1976 after admitting eight offences of deception, forgery and unlawful possession of the drug. He claimed he had taken drugs because he was depressed and unhappy about work.

Dame Janet said: ``There is no obvious reason why Shipman should have been depressed, anxious or deeply unhappy in the 1970s.''

Dame Janet said she accepted the psychiatrists' view that Shipman might have had ``all manner of underlying problems''.

``It seems to me that whatever problem it was that drove him to pethidine addiction in the 1970s was almost certainly never resolved and probably became a permanent part of his make-up,'' she said.

``The psychiatrists say that a person who has one addiction is quite likely to be the subject to other forms of addiction.

``I think it likely that whatever it was that caused Shipman to become addicted to pethidine also led to other forms of addictive behaviour.''

Asked if a motivation would ever become clear, Dame Janet said: ``The short answer, I think, is no.''

Dame Janet said there was no evidence that Shipman was motivated by monetary gain. Very few of his patients left him cash. It appears greed was Shipman's motive when he killed his final victim, 81-year-old Kathleen Grundy, and forged her will to leave all her property to the doctor.

It was the forgery - described by Dame Janet as ``grossly incompetent'' - which led to Shipman's downfall.